We have previously described a range of improvements in touch sensing systems of the type which project a sheet of light just above a surface or display, for example to provide an electronic whiteboard. Examples of such improvements can be found in our earlier filed WO2013/144599.
We now describe further improvements to pen-based touch systems which are particularly, but not exclusively applicable to ‘sheet of light’ touch sensing systems. In the later description we refer to pens but the skilled person will appreciate that in this specification ‘pen’ is to be interpreted broadly covering other hand held devices usable for indicating a position on a surface such as a whiteboard including, for example, wands and the like.
A sheet of light based touch sensing system typically uses a camera to capture an image of light scattered by a pen intersecting the sheet. This imposes an image capture and processing delay greater than the shutter speed, for example greater than 1/60 of a second. In addition, for this type of system a pen is invisible if it is hovering just above the sheet of light. A similar effect can occur, for example, when writing, when the scattered light from a pen (the ‘blob’ in the image captured by a camera of the scattered light) disappears and reappears during the writing process. A touch detection system may incorporate a touch tracking process such as a tracking filter, and it would be useful for such a process to maintain an approximate location for a pen even when the pen was not breaking the light sheet.
Further issues arise when multiple pens are used simultaneously, and/or when one or more of the light scattering regions in a touch image is not a pen but a finger. More particularly, it is useful to be able to identify individual pens during use, in particular when intersecting the light sheet, that is when not ‘hovering’, so that different pens can be given different functions, for example different colors. We have previously described techniques which enable a user to ‘write’ on a touch sensitive surface with different color pens (ibid), but nonetheless improvements are desirable.